1. Field of the Invention
The present invention relates to a multimode optical transmission apparatus and a multimode optical transmission system for transmitting an optical signal via a multimode optical transmission line, and more particularly relates to a multimode optical transmission apparatus and a multimode optical transmission system for reducing a deterioration of a noise characteristic and a distortion characteristic caused by an interference between modes.
2. Description of the Related Art
For example, in D. R. Hjelme, and A. R. Mickelson, “Microbending and modal noise”, Applied Optics, Vol. 22, No. 23, Dec. 1, 1983, pp. 3874-3879 (hereinafter, referred to as “Non-patent Document 1”), an optical transmission system using a conventional multimode optical fiber is disclosed. FIG. 9 is a block diagram illustrating an exemplary structure of the conventional optical transmission system disclosed in Non-patent Document 1. As shown in FIG. 9, the conventional optical transmission system includes an electric-optic conversion section 501 and a photoelectric conversion section 502, which are connected to each other via a multimode optical transmission line 503.
An electrical signal is inputted to the electric-optic conversion section 501. The electric-optic conversion section 501 converts the inputted electrical signal into an optical signal and outputs the optical signal to the multimode optical transmission line 503. For the multimode optical transmission line 503, a multimode optical fiber is used. The optical signal propagates in the multimode optical transmission line 503 to the photoelectric conversion section 502. The photoelectric conversion section 502 converts the inputted optical signal into an electrical signal and outputs the electrical signal. In general, a semiconductor laser such as a Fabry-Perot laser is used for the electric-optic conversion section 501. A PIN photodiode, an avalanche photodiode or the like are used for the photoelectric conversion section 502.
In such optical transmission system using the multimode optical fiber, the multimode optical fiber, which has a large core diameter, can be easily connected to a light source, and therefore the system can be constructed at a lower cost than a system using a single mode optical fiber. Because of such advantages, the optical transmission system using the multimode optical fiber is currently in widespread use for systems, such as an office LAN which performs a transmission within a relatively short distance.
However, the optical transmission system using the multimode optical fiber has a plurality of propagation modes in its optical transmission line, so that a transmission characteristic is deteriorated. That is, in the optical transmission system using the multimode optical fiber, some of the propagation modes interfere with each other, which causes a noise called a modal noise. Here, the modal noise refers to a noise occurring due to a temporal change of a speckle pattern caused by the interference between the modes. Further, a bending of the optical fiber or a gap between connectors substantially deteriorates the transmission characteristic.
As described above, when the optical transmission is performed using the multimode optical fiber, the transmission characteristic is deteriorated due to the interference between the modes. Particularly, when the optical transmission line has a small number of modes, or when a light source has a narrow line width, the possibility for the interference between the modes is increased, thereby significantly deteriorating the transmission characteristic. For example, when a surface emitting laser and a 1.3 μm band single mode optical fiber are used as the light source and the optical transmission line, respectively, the interference between the modes is increased, thereby significantly deteriorating the transmission characteristic. A wavelength of the surface emitting laser is typically in a short wavelength region (that is, 850 nm). When the surface emitting laser has such a short wavelength and the single mode optical fiber is used as the optical transmission line to perform optical transmission, the number of modes is reduced to about two or three.
Further, it is assumed that, as the light source connected to the single mode optical fiber, a single mode surface emitting laser having a Gaussian distribution of far field patterns is used. In this case, the optical transmission line has a small number of modes and the light source has a narrow line width, so that the interference between the modes significantly deteriorates the noise characteristic and the distortion characteristic of the multimode optical transmission line.